In the field of double-barrelled trapshooting guns and hunting guns with the barrels positioned either side by side or one over the other, there are already known several types of trigger mechanisms. These mechanisms comprise, generally, a pair of hammers, corresponding to the two barrels of the gun, and a single trigger for controlling the successive disengagement of the two hammers. Each conventional trigger mechanism has its own characteristic features and its members are mounted on an underguard which is applicable to the lower portion of the breech of the gun, in correspondence with a suitable opening or slit. This allows the assembling and disassembling, as a unit, of the trigger mechanism.
However, the mounting and dismounting of the trigger mechanism, in practice, is neither simple nor user friendly, as one would indeed desire. Furthermore, the trigger of these mechanisms cannot be positioned longitudinally, in answer to the user's needs, without avoiding the possibility of an uncontrolled displacement or involuntary opening of the trigger itself.